Implantable cardiac defibrillator (ICD) systems are commonly used today to prevent sudden cardiac deaths. The primary components of all existing ICD systems include an automatic monitoring and detection mechanism, a capacitor system, a battery system and control circuitry for detecting a ventricular arrythmia and controlling delivery of a high voltage capacitive discharge electrical countershock in response to the detected arrythmia by charging and then discharging the capacitor system. To achieve successful defibrillation, the ICD system must deliver a high voltage electrical countershock with an initial voltage of greater than about 500 to 600 volts.
Most existing ICD systems are capable of delivering a maximum countershock of up to 700 to 750 volts having a total energy of between 31 to 44 joules. The capacitor system is a critical element of the ICD system, both in terms of how effective the ICD system is and how small the ICD system is. The capacitor component is the largest single component in the ICD. By definition, a capacitor is comprised of two conductive surfaces separated by an insulating material. The insulating material is known as the "dielectric" of the capacitor. When the two surfaces of the capacitor are oppositely charged by a voltage source, such as the battery in an ICD system, electrical energy is effectively stored by the capacitor in the polarized dielectric. The capability of the capacitor to store an electrical charge is the capacitance value of capacitor. For a given dielectric material, the thinner the dielectric, the higher the capacitance value. A thinner dielectric also decreases the overall size of the capacitor. Unfortunately, there are limits as to how thin a dielectric can be due to the fact that very thin dielectrics will break down under high voltages as there is simply an insufficient amount of insulation material between the conductive surfaces to withstand the high voltages.
When all of the requirements for an ICD system are considered, the aluminum oxide electrolytic capacitor (also known as a photo flash capacitor) has proven to be the best capacitor technology for use in ICD systems to date. The aluminum oxide dielectric can be made very thin because the dielectric oxide is essentially grown on the conductive surfaces of a very thin sheet of aluminum that has been etched to increase its effective surface area. As a result, aluminum oxide electrolytic capacitors have higher energy densities (typically 1.7 to 1.8 joules per cc) than other types of capacitor technologies (typically much less than 1.5 joules per cc). Due to the nature of the aluminum oxide dielectric, electrolytic capacitors are typically limited to a maximum rated charging voltage in the range of approximately 350 to 375 volts. Beyond 375 volts, electrolytic capacitors begin to suffer from significant leakage current across the dielectric. This leakage current increases rapidly as the voltage is increased and charging of the electrolytic capacitor will cease when the leakage current equals the charge current. As a result, most existing ICD systems utilize two electrolytic capacitors in series, each being charged with approximately 375 volts, which are then discharged to deliver the high voltage shock to the myocardium having a maximum voltage of approximately 750 volts.
Although electrolytic capacitors are used in most existing ICD systems in order to take advantage of their excellent capacitance to volume ratio, electrolytic capacitors suffer from several major drawbacks. First, as stated above, the useful charging voltage for electrolytic capacitors is limited to approximately 375 volts due to the leakage current encountered at higher charging voltages. This requires that two electrolytic capacitors be used which increases the number of components within the ICD system.
Another significant disadvantage of electrolytic capacitors is the degradation of the oxide dielectric over time. Although the dielectric degrades, it can be reformed by periodic charging to full voltage. On a monthly or quarterly basis, the capacitor system will need to be charged to its full voltage. In early ICD systems, this requirement necessitated the patient's periodic return to the hospital to accomplish the reforming of the capacitor system. Later ICD systems have used automatic reforming of the electrolytic capacitors from the internal battery system on a periodic basis. This practice is wasteful of valuable energy in ICD system that only has a finite and depletable source of power.
Still another drawback of the electrolytic capacitors is that a substantial portion of the energy density advantage over other capacitive technology is lost to packaging inefficiencies within the ICD system as a result of the cylindrical packaging shape that is required of electrolytic capacitors. When the lost volume of fitting a cylindrical volume into a rectangular volume is factored into the energy density calculations, the energy density for electrolytic capacitors is effectively only about 1.3 to 1.4 joules per cc.
The highest density energy storage capacitors available are high voltage thin film capacitors which are capable of densities on the order of four joules per cc. In thin film capacitors, the dielectric is a very thin polymer film that is formed mechanically through high precision rolling operations. Conductive layers of aluminum are then deposited on each surface for the polymer film. One advantage of thin film capacitors are that they have very high breakdown voltages and very good charge retention. As a result, if a polymer thin film capacitor were used in an ICD system, there would be no need to use two separate capacitors to achieve the initial discharge voltage required for defibrillation. Nor would there be any need to reform the capacitor due to breakdown of the dielectric. They are also manufacturable in almost any shape.
Unfortunately, to achieve densities in the range of four joules per cc, the thin film must be charged up to high voltages on the order of 2000 volts or greater. The highest densities do not occur until around 4000 volts. Unfortunately, these voltages would damage the heart. For instance, myocardial tissue resistance between any two implanted discharge electrodes has been found to be about 50 ohms on average. Using this average resistance value, the peak current of an electrical countershock delivered from a capacitor charged to 2000 volts would be 40 amps. It is known that peak currents in excess of about 30 amps during delivery of an electrical countershock can lead to tissue destruction in the heart in a zone beginning from the center of the electrical field and extending outward. High peak currents also stun tissue extending radially outward from the border of the destruction zone for some additional distance.
Another problem with discharge voltages of 2000 volts or greater, for a given level of energy storage is that the capacitor size would be such that the pulse width would be inefficient. For example, to store 25 joules in a film capacitor of 2100 volts would require a capacitance of 11.3 microfarads. Such a capacitor would have a time constant of about 0.5 milliseconds when delivering a charge to a 50 ohm heart load. Such a pulse width is significantly shorter than the optimal duration for defibrillation which is on the order of two to five milliseconds.
One approach to using thin film capacitors in an ICD is taught in U.S. Ser. No. 08/342,637 filed Nov. 21, 1994 which is a continuation of 08/166,212, filed Dec. 13, 1993 entitled IMPLANTABLE CARDIOVERTER DEFIBRILLATOR EMPLOYING POLYMER THIN FILM CAPACITORS, which is assigned to the assignee of the present application and the disclosure of which is herein incorporated by reference. This application has the drawback that the body or the heart may be exposed to the full voltage from the primary film capacitor even though the average voltage is moderate due to chopping.
Another possible approach for film capacitors is taught in U.S. Pat. No. 5,391,186 entitled METHOD AND APPARATUS FOR UTILIZING SHORT TAU CAPACITORS IN AN ICD, which is assigned to the assignee of the present invention and which is herein incorporated by reference. Once again, this system allows the body or the heart to be exposed to the full voltage of primary film capacitor even though the average voltage is moderated due to chopping.
While the use of electrolytic capacitors for ICD systems has allowed for the creation of practical implantable devices that can deliver effective electrical countershocks, there are inherent limitations of the electrolytic capacitors which hinder further reduction in the size of ICD systems by reducing the size of the capacitor systems necessary to deliver the capacitive discharge electrical countershock. Therefore, it is desirable to provide an implantable cardioverter defibrillator system which could employ the use of capacitor technology other than the electrolytic capacitors. In addition, it would be advantageous to provide an implantable cardioverter defibrillator system that could take advantage of the higher charging voltages available with thin film capacitors while protecting the body or heart from being exposed to the full voltage of these higher charging voltage capacitors.